There are a number of methods for organizing and cataloging your iPhoto '11 library. You can dump photos into albums, and you can set criteria for ever-evolving smart albums. You can use the Faces and Places features to sort by people or locations. You can flag photos. And you can use Events to sort photos taken during a certain time period.
The above methods are very effective for helping you locate photos in a sprawling iPhoto library, but for a more granular approach, I suggest you try keywords. Keywords act like tags, which help you quickly search for a particular photo.
Before you can start adding keywords, you first must click on View from the top menu and click on Keywords so that the check mark appears next to it. This will show the Keywords menu option when you click on the Info button in the toolbar in the lower-right corner of iPhoto.
Now, to add a keyword, highlight a photo or photos (you can also select an Event) and click the Info button. From the Info pane, you'll see the Keywords field between Faces and Places. Click on "Add a keyword..." and type as many keywords as you'd like.
To search your library by keyword, click the Search button in the lower-left corner of iPhoto. Words used in the title or description fields for a photo or an event also act as keywords.
To edit the keywords you've used, go to Window > Manage My Keywords (or use the keyboard shortcut Command-K). From the resulting pop-up window, you'll see a list of your keywords. You can drag keywords into the Quick Group field, which assigns a letter to a keyword, letting you add it with a single keystroke. Click the Edit Keywords button, and you can add or remove keywords, rename them, and assign a letter for a shortcut.
If you find yourself constantly struggling to locate particular photos in iPhoto, a little work upfront by assigning keywords will save you a lot of time blindly scrolling and searching down the road.